Protein function can be modified and improved in vitro by a variety of methods, including site directed mutagenesis (Moore et al, 1987) combinatorial cloning (Huse et al, 1989; Marks et al, 1992) and random mutagenesis combined with appropriate selection systems (Barbas et al, 1992).
The method of random mutagenesis together with selection has been used in a number of cases to improve protein function and two different strategies exist. Firstly, randomisation of the entire gene sequence in combination with the selection of a variant (mutant) protein with the desired characteristics, followed by a new round of random mutagenesis and selection. This method can then be repeated until a protein variant is found which is considered optimal (Moore et al, 1996). Here, the traditional route to introduce mutations is by error prone PCR (Leung et al, 1989) with a mutation rate of ≈0.7%.
Secondly, defined regions of the gene can be mutagenized with degenerate primers, which allows for mutation rates up to 100% (Griffiths et al, 1994; Yang et al, 1995). The higher the mutation rate used, the more limited the region of the gene that can be subjected to mutations.
Random mutation has been used extensively in the field of antibody engineering. In vivo formed antibody genes can be cloned in vitro (Larrick et al, 1989) and random combinations of the genes encoding the variable heavy and light genes can be subjected to selection (Marks et al, 1992). Functional antibody fragments selected can be further improved using random mutagenesis and additional rounds of selections (Hoogenboom et al, 1992).
The strategy of random mutagenesis is followed by selection. Variants with interesting characteristics can be selected and the mutagenized DNA regions from different variants, each with interesting characteristics, are combined into one coding sequence (Yang et al, 1995). This is a multi-step sequential process, and potential synergistic effects of different mutations in different regions can be lost, since they are not subjected to selection in combination. Thus, these two strategies do not include simultaneous mutagenesis of defined regions and selection of a combination of these regions. Another process involves combinatorial pairing of genes which can be used to improve e.g. antibody affinity (Marks et al, 1992). Here, the three CDR-regions in each variable gene are fixed and this technology does not allow for shuffling of individual CDR regions between clones.
Selection of functional proteins from molecular libraries has been revolutionized by the development of the phage display technology (Parmley et al, 1987; McCafferty et al, 1990; Barbas et al, 1991). Here, the phenotype (protein) is directly linked to its corresponding genotype (DNA) and this allows for directly cloning of the genetic material which can then be subjected to further modifications in order to improve protein function. Phage display has been used to clone functional binders from a variety of molecular libraries with up to 1011 transformants in size (Griffiths et al, 1994). Thus, phage display can be used to directly clone functional binders from molecular libraries, and can also be used to improve further the clones originally selected.
Random combination of DNA from different mutated clones is a more efficient way to search through sequence space. The concept of DNA shuffling (Stemmer, 1994) utilises random fragmentation of DNA and assembly of fragments into a functional coding sequence. In this process it is possible to introduce chemically synthesised DNA sequences and in this way target variation to defined places in the gene which DNA sequence is known (Crameri et al, 1995). In theory, it is also possible to shuffle DNA between any clones. However, if the resulting shuffled gene is to be functional with respect to expression and activity, the clones to be shuffled have to be related or even identical with the exception of a low level of random mutations. DNA shuffling between genetically different clones will generally produce non-functional genes.